This invention relates to an electroluminescent (EL) device and more particularly to an alternate current (AC)-drive thin-film EL device and a method of manufacturing the same.
The AC-drive thin-film EL device has excellent brightness and stability characteristics and is widely used for various types of displays. A typical double insulated type thin-film EL device comprises a transparent glass substrate and multi-layers of thin films of a transparent electrode, a thin-film first insulator layer, a thin-film luminescent layer, a thin-film second insulator layer and a thin-film rear electrode, which are sequentially laminated on the substrate, as described in SID 74 Digest of Technical Papers; pp. 84 to 85. The first and second insulator layers may be transparent dielectric thin films of Y.sub.2 O.sub.3, T.sub.a2 O.sub.5, Al.sub.2 O.sub.3, S.sub.i N.sub.4, B.sub.a T.sub.i O.sub.3 or S.sub.r T.sub.i O.sub.3 of 0.2 to 1 thickness fabricated by a sputtering or vacuum evaporation method. These insulator layers aim at improving luminescence and stability of the EL device operation characteristics by limiting electric current passing through the luminescent layer as well as at enhancing the reliability of the EL device by protecting the luminescent layer from contamination by harmful ions and/or moisture.
The above mentioned devices, however, have several practical problems. More particularly, they are defective in that dielectric breakdown cannot be eliminated completely over a wide area which reduces production yield, and that the driving voltage applied on the device for emitting light becomes inevitably high as the voltage is divided and applied on the insulator layers. In order to solve the former problem of dielectric breakdowns, it is necessary to employ materials having superior dielectric characteristics for the insulator layers. For the latter problem, it is preferable to increase the capacity of the insulator layers in order to minimize the split of the voltage applied on the insulator layers.
The electric current passing through the luminescent layer is substantially proportionate to the capacity of the insulator layers according to the operating principle of such AC drive EL device. Therefore, increasing the capacity of insulator layers is critical for reducing driving voltage as well as for enhancing brightness. In short, the insulator layers should have a great resistance against dielectric breakdowns and a large capacity. As an index for evaluating the merits of insulating materials, a product of a dielectric constant (.epsilon.) and a dielectric breakdown field (Eb.d.) is widely used. The minimum value of .epsilon..Eb.d. required for practical use is about three times as much as the .epsilon.Eb.d. value (ca. 1.3 .mu.C/cm.sup.2) of ZnS luminescent layer. (Refer to IEEE Transactions On Electron Devices ED-24, pp. 903 to 908 (1977).) If an insulator material has an extremely large Eb.d. value, it may realize an insulator layer having a large capacity by taking the form of extremely thin films even if the value .epsilon. is small. But in practice, it is very difficult to completely eliminate pin holes or adhesion of particles over a large area which is required for a flat display or a surface-area light source. For such practical reason, it is not appropriate to employ insulator layers of thickness of several 100 .ANG. or less.
Use of thin films of high dielectric constant is being reviewed from the aformentioned point of view. For instance, PbTiO.sub.3 film fabricated by the sputtering method is used as an insulator layer in order to reduce driving voltage. (Refer to IEEE Transaction On Electron Devices ED-28, pp. 698-702 (1981).) The sputtered film of PbTiO.sub.3 exhibits 0.5 MV/cm in dielectric strength at the maximum relative dielectric constant of 190, but the substrate temperature required during sputtering is as high as ca. 600.degree. C., and this is not practical.
There has been known SrTiO.sub.3 film fabricated by sputtering which can show fairly good .epsilon..Eb.d. value. (Refer to Japan Display 1983, pp. 76 to 79 (1983).) The sputtered film of SrTiO.sub.3 has the relative dielectric constant of 140, dielectric breakdown voltage of 1.5 to 2 MV/cm, and .epsilon..Eb.d. value of 19 to 25 .mu.C/cm.sup.2. That value is higher than that of PbTiO.sub.3 or 7 .mu.C/cm.sup.2. However, in the case of SrTiO.sub.3 film, the substrate temperature during sputtering needs to be as high as 400.degree. C., and moreover it reduces ITD transparent electrodes to blacken it during sputtering. It is detrimental also in that it cannot achieve a strong adherion with ZnS layer. Further, thin film EL devices using these insulator layers of relatively high dielectric constants tends to cause catastrophic propagating breakdowns, which are fatal in practice, rather than the breakdown of self-healing types which are completed to leave only the small problems.
As described in the foregoing, it is practically impossible to employ insulating thin films of high dielectric constant and .epsilon..Eb.d. value and still to achieve low working voltage, high brightness, desirable luminiscent characteristics and stability against dielectric breakdowns.
Due to the thermal processing required for improvement in stability and characteristics of EL devices, an expensive glass substrate should be used which is alkali-free and have high softening point. This factor inevitably increase the cost of thin-film EL devices. Even if such expensive glass is used, the temperature in the process should be limited to less than 600.degree. C. The resistivity of ITO film used as a transparent electrode is not small enough. If the thickness of ITO film is increased in use, possibility of dielectric breakdown occurrence increases along ITO pattern edge. The thickness therefore should be limited to 0.2 .mu.m or less. The electrode resistance cannot be heretofore reduced to a satisfactory level, presenting a problem in realization of large area and large capacity displays.
Instead of using a glass plate as a starting substrate for forming thin-film structure, there has been proposed to use a high dielectric constant sintered ceramic substrate are formed a luminescent layer and a thin-film transparent electrode by thin-film fabrication technology. And a thin-film rear electrode is formed on the bottom surface of the sintered ceramic substrate by thin-film fabrication technology. (Refer to Japanese Patent Application No. 114461/82 which was published under Unexamined Patent Publication No. S-268/84.) This structure is advantageous in that it achieves low working voltage and high stability against dielectric breakdowns. However, the thickness of the high dielectric constant sintered ceramic substrate is selected as thin as 0.05 to 0.2 mm so as not to decrease coupling capacitance and thus results in small mechanical strength. For this reason, it can only realize an extremely small area EL device. If a large area display device is required, plural EL devices of a small area must be mounted on a rigid supporting member such as an alumina ceramic plate to have desired display patterns. Electric connection in such a case should be made by aligning pieces of EL devices with electrodes which are already formed on the alumina ceramic plate, requiring cumbersome works in manufacture. Furthermore, such fragile substrate should be handled with great care.